Panels have a fixed width of 17em (272 pixels) which is narrow enough to
still show some of the page contents when open to make clicking out to
close easy, and still looks good on wider tablet or desktop screens. The
styles to set widths on panels are fairly complex but these can be
overridden with CSS as needed.

Just a side note here - the proper form for this is Developing JQuery Mobile. That forum is for discussion the actual development of JQuery Mobile itself - including enhancements.

Unfortunately not many people go there and there doesn't seem to be much activity. The best place to request these features is on the github. You can create a new enhancement request which the JQM team will/should take a look at.